The general background of this invention is the field of X-ray spectral computed tomography (CT). In a CT system an X-ray source emits X-ray radiation. The emitted radiation traverses an examination region with a subject or object located within and is detected by a detector array opposite the X-ray source. The detector array detects the radiation traversing the examination region and the subject and generates projection data, e.g. raw detector data or projection images. A reconstructor processes the projection data and reconstructs a volumetric image of the subject or object. X-ray Spectral CT is an imaging modality that extends the capabilities of a conventional CT system. Dual-Energy (DE) CT, which is a specific configuration of spectral CT, utilizes two attenuation values acquired at two photon energies to solve the photoelectric and Compton contribution that consists of the mass attenuation coefficient of a material, and thus to identify an unknown material by its value of photoelectric and Compton contribution. This scheme works especially well in materials such as iodine that has k-edge energy close to the mean value of a diagnostic energy range. Because any two linearly independent sums of two basis functions span the entire attenuation coefficient space, any material can be represented by a linear combination of two other materials, so called basis materials, such as water and iodine. The basis material images provide new applications such as monochromatic image, material cancellation image, effective atomic number image and electron density image. There are several approaches to perform dual energy CT acquisition such as dual-source, fast kVp switching, and dual-layer detector configurations. In addition, quantitative imaging is one of the current major trends in the medical imaging community. Spectral CT supports this trend, as the additional spectral information improves the quantitative information that can be measured about the scanned object and its material composition.
Such Multi Material Decomposition (MMD) is becoming a fundamental task for many clinical applications, in which the goal is often to characterize, detect and/or quantify the amount of a given material. MMD for DE CT is very challenging, because as discussed above, as there are only two acquired energy attenuations in principle two materials can be decomposed accurately. By further constraining the problem, three materials can be decomposed. However, utilizing DE CT for decomposition for more than three materials is an ill-posed problem.
US2008/0253504A1 relates to a CT system for determining the quantitative material concentrations of the components, such as bone, blood, contrast agent, in a region of interest of an object, such as a patient. To provide a CT system which improves the quality and explanatory power of quantitative material decomposition, a CT system is proposed comprising: a scanning unit having a radiation source and a detector unit for acquisition of spectral CT projection data from said region of interest; a modeling unit for obtaining a photoelectric effect projection data set and a Compton effect projection data set by decommodeling unit posing said spectral CT projection data set by means of respective models of photoelectric effect and Compton effect; a reconstruction unit for reconstructing a photoelectric effect image and a Compton effect image of said region of interest from said photoelectric effect projection data set and Compton effect projection data set; a processing unit for determining the concentrations of said components in said region of interest by solving a system of equations obtained by equating said photoelectric effect image data with the accumulated products of said concentrations and photoelectric attenuation coefficients for said components and equating said Compton effect image data with the accumulated products of said concentrations and Compton attenuation coefficients for said components.
Further information relating to MMD can be found in the following documents: Mendonça, Paulo R S, et al. “Multi-material decomposition of spectral CT images.” SPIE Medical Imaging. International Society for Optics and Photonics, 2010; Long, Yong, and Jeffrey A. Fessler. “Multi-material decomposition using statistical image reconstruction in X-ray CT.” Proc. 2nd Int. Mtg. on image formation in X-ray CT (2012): 413-6; and Mendonça, Paulo R S, Peter Lamb, and Dushyant V. Sahani. “A flexible method for multi-material decomposition of dual-energy CT images.” Medical Imaging, IEEE Transactions on 33.1 (2014): 99-116.